1. Field of the Invention
The present invention relates to an antenna device to be used in a milliwave band, and a transmitting and receiving unit using it.
2. Description of the Related Art
In a radar using a microwave or a millimeter wave, the monopulse system has been employed as one of angle measurement systems to measure the azimuth of a target.
FIG. 13 is a schematic representation of the beam pattern of a radar to achieve the angle measurement by the amplitude-comparison monopulse system. In the figure, TX1 denotes a transmission beam to be transmitted from a front end, and RX1 and RX2 denote reception beams to indicate the direction characteristic of a reception antenna to make reception at a front end. FIG. 14 is a schematic representation of the angle measurement based on two reception signals. FIG. 14A shows the angle on X-axis, and the reception signal intensity on Y-axis. FIG. 14B shows the patterns of the sum component and the difference component of the two beams. The S-shaped characteristic of the error voltage to the angle can be obtained as indicated in FIG. 14C if the ratio of the difference component to the sum component is extracted as the angle error voltage signal .epsilon.. An approximately straight part of the S-shaped characteristic corresponds to the angle range between the axes of the two reception beams RX1 and RX2 as indicated in FIG. 13. Thus, the azimuth of the target can be detected from the extracted angle error voltage for this detection range.
In an example, when a vehicle is traveling on the road, the above-mentioned monopulse system is applicable to measure the distance and the relative speed to another vehicle traveling forward or backward, and to measure its azimuth, the angle measurement range becomes a problem according to some service intention of a radar. That is, in a case of the monopulse system of the above-mentioned construction, the angle of expansion between the axes of the two reception beams is the angle measurable range. If individual beams are made wider in angle to increase the range, the gain is decreased accordingly, and the measurable distance is reduced. If the angle of expansion between the axes of the two beams is increased without being wider in angle, the angle measurement resolution is degraded.